1. FIELD OF THE INVENTION
The invention relates to a laser Doppler anemometer having means for splitting the laser radiation into a plurality of partial bundles of rays, a common optical image-forming system for illuminating the measuring volume and for receiving the back-scattered signal radiation, and a detector provided in the optical path of the scattered signal radiation.
2. DESCRIPTION OF THE PRIOR ART
In a known laser Doppler anemometer of this type as disclosed in published German patent specification DT-OS No. 2 043 290, the light detector, for example a photomultiplier, including the associated optical system, lies on the optical axis of the image-forming system, behind the image-forming system, seen from the measuring volume. It is true that this arrangement renders possible a compact construction of the optical system, which is easy to adjust, but the receiving optical system associated with the detector, together with the associated operational controls and supporting members, is constricted in space, because the said parts are within an area bounded by the two illuminating beams. A further drawback of what is known consists in that the common image-forming system cannot be fully utilized.
In another known device (Proc. of 16th Int. Aerospace Instr. Symposium, May 1970, pages 14 - 26, particularly FIG. 7) the reverse method was adopted: There, the light scattered backwards from the measuring volume passes through the marginal zone of the illuminating and receiving lens. Thus this device also has the disadvantage indicated above. In addition, a further drawback is to be seen in the fact that the light source cannot lie on the optical axis of said lens, which leads to difficulties in adjustment on the one hand and does not permit rotation of the beam splitter for the measurement of various velocity components orthogonal to the direction of incidence of the illuminating beam, on the other hand.